Method of increasing the conductive path of steel shot



United States Patent 3,277,953 METHOD OF INCREASING THE CONDUCTIVE PATH OF STEEL SHOT Gerald Fisher, Wilmette, Ill., assiguor to Stanray Corporation, Chicago, Ill., a corporation of Delaware No Drawing. Filed Nov. 19, 1963, Ser. No. 324,863 4 Claims. (Cl. 165-1) This is a continuation-in-part of application Serial No. 120,498, filed June 29, 1961, and now abandoned.

This invention relates to a form of material for use as a filling and packing of enclosed spaces for the purpose of aiding and increasing the transmission of heat through said enclosed spaces.

If it is desirable to conduct heat away from an object, especially an irregularly shaped object of complicated cross section within a container, it is possible to pour small (approximately .020 inch in diameter) shot-like or granular particles into the container and about the object, and said particles will flow therein until the entire space not occupied by the object is completely filled with said particles. These small shot-like particles will flow, as a liquid would, but will not leak out through small cracks or openings in the container walls.

The utilization of small shot-like steel particles (low carbon and medium carbon steel) as a heat transfer medium is interesting because of the ability of these particles to flow as a liquid would, and also because of a physical and chemical change which takes place in the entire mass once the mass has been subjected to a temperature of 750 F. in the presence of an oxidizing atmosphere.

By steel is meant the usual commercial form of iron containing carbon in any amount up to about 1.7% as an essential alloying constituent. For commercial purposes steel containing less than 0.25% carbon is referred to as mild or soft steel; that containing 0.25 to 0.60% is referred to as medium steel; and that containing more than 0.60% carbon is referred to as hard steel. The classification of steel is in grades of hardness. The steel shot of this invention is of the mild or medium steel.

When low or medium carbon steel shot of the above character is heated to a temperature of 750 F. in the presence of an oxidizing atmosphere (the ordinary furnace atmosphere), a coating of oxide forms on the surface of the shot. This is a magnetic oxide (Fe O and forms on the surface of each shot-like particle.

When this film forms, it compacts the shot into an almost solid mass within the container and about the object from which heat is to be conducted, and provides a greater path for thermal conductivity.

The chemical change referred to is the growth of an oxide skin on each of the small particles. The oxide skins of adjacent particles are not only in contact with each other but also have entire sections in common so that the area of contact and the path for transmission of heat has been greatly increased.

The physical change mentioned above refers to the fact that once the growth and intergrowth of oxide skins take place, the steel particles are now a loosely held, close knit, single mass. The temperature of 750 F., at which the above changes take place for steel, is below the temperature at which sintering or fusion of the steel particles is normally expected to take place.

After the above physical and chemical changes take place, the mass of particles exhibits a higher coefficient of thermal conductivity at temperatures below 750 F. than that which it exhibited prior to being heated to the temperature at which the above changes took place.

A practical application of the phenomenon is illustrated in the following:

3,277,953 Patented Oct. 11, 1966 An object such as a spent fuel element from an atomic reactor, from which it is desirable to provide a means of carrying away heat, is placed within a container, such as a cask. Said object is then completely surrounded by small, spherical, shot-like steel particles, by pouring same into the container to thereby completely fill the container. The entire container contents are then heated in the presence of an oxidizing atmosphere to the aforesaid temperature necessary to effect the required changes in the steel particles.

After the outside heat source is removed, the mass now filling the container will exhibit a thermal conductivity greatly increased over that which would have been exhibited had no outside heat source been applied and no physical and chemical changes taken place. The increased thermal conductivity will be effective at temperatures well below that which caused the physical and chemical changes previously described.

The application of the invention is especially useful as a heat conducting packing about a spent fuel element from an atomic reactor being shipped in a cask to a reprocessing plant. Spent fuel elements when removed from the reactor are still very radioactive and also thermally hot. The cask must therefore be able to confine the radioactivity during shipment and consequently the cask Walls are very thick and dense. It is also highly desirable to dissipate the heat as rapidly as possible. By pouring applicants small spherical particles of steel shot into the cask until all air space about the fuel elements is filled, a path of heat transfer through the spheres to the cask walls is provided, and through the cask walls to the outside atmosphere. The atmosphere within the cask is an oxidizing atmosphere, the same as the outside atmosphere.

When the steel shot are placed in the cask, they function as a heat conductor, and since the fuel elements which they surround generate heat, the temperature of the shot is raised to about 750, at which point the aforesaid chemical and physical changes take place by the formation of a film of scale over the surfaces of the shot.

There is thus a growth and an intergrowth of the oxide skins or scale which take place, causing an extreme crowding together of the oxide coated shot much more than a point to point contact of the individual spheres, which become a single mass, greatly increasing the rate of thermal conductivity.

What I claim is:

1. The method of increasing the thermal conductivity of small solid steel shot, of placing said shot in a container surrounding an object from which heat is to be conducted, and heating said shot to approximately 750 F. to cause growth 'of scale and greater contact area between the particles of shot to thereby provide a greater heat conductive path therethrough, and more rapid dissipation of heat from said object.

2. The method of increasing the thermal conductivity of small steel shot, of placing said shot in a container about an object from which heat is to be conducted, and heating said shot to about 750 F. to cause scale to form on the surface thereof and thereby increase the pressure and contact area between the particles providing a greater heat conductive path therethrough, and more rapid dissipation of heat from said object.

3. The method of increasing the coefficient of thermal conductivity of small granular steel shot at temperatures below that required to effect scale growth, of pouring said material into a container about an object from which heat is to be extracted until the material fills the space not occupied by the object, and heating said shot to a temperature of about 750 F. in the presence of an oxidizing atmosphere to effect the growth of scale on said shot, thereby causing greater contact area between the particles of shot, providing a greater heat conductive path therethrough, and a more rapid dissipation of heat from said object.

4. The method of increasing the rate of thermal conductivity from a spent fuel element from an atomic reactor in a shipping cask, of pouring small steel shot into said cask about said fuel element until the shot fills the space not occupied by said element, heating said shot to a temperature of about 750 F. in the presence of an oxidizing atmosphere to effect growth of scale on said shot, thereby causing greater contact area between the particles of shot, providing a greater heat conductive path therethrough and a more rapid dissipation of heat from said element.

4 References Cited by the Examiner FOREIGN PATENTS 7/1959 France. 1/1933 Great Britain.

ROBERT A. OLEARY, Primary Examiner. CHARLES SUKALO, Examiner. 15 M. A. ANTONAKAS, Assistant Examiner. 

1. THE METHOD OF INCREASING THE THERMAL CONDUCTIVITY OF SMALL SOLID STEEL SHOT, OF PLACING SAID SHOT IN A CONTAINER SURROUNDING AN OBJECT FROM WHICH HEAT IS TO BE CONDUCTED, AND HEATING SAID SHOT TO APPROXIMATELY 750* F. TO THE CAUSE GROWTH OF SCALE AND GREATER CONTACT AREA BETWEEN THE PARTICLES OF SHOT TO THEREBY PROVIDE A GREATER HEAT CONDUCTIVE PATH THERETHROUGH, AND MORE RAPID DISSIPATION OF HEAT FROM SAID OBJECT. 